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later philosophers, as d'Alembert, Euler, Simpson, Silvabelle, Frisi, Walmcoley, Laplace.
In article VI, on the diminution of the obliquity of the ecliptic, it is shown that this decrease would be at the rate of about 50" in a hundred ycars, if we acknowicde the accuracy of the ancicnt observations, since it is stated by Ptolemy at 23° 51', and its present obliquity is 23° 28'. But as the first quantity is probably too great, the rate is stated with more accuracy at 35" per 100 years. This gradual diminution is traced from the ancients, through the Arabs, and the more modern astronomers, to the present time. It is shown that it arises chieily from the actions of the planets Jupiter and Venus, the effects of which are computed as near as can be by Euler and Laplace; and whence it appears that the decrease of the obliquity will not continue perpetually, or till the ecliptic coincide with the equator, but only to a certain term; after which it will increase again to another term, so as to re-acquire its greatest quantity. The obliquity is also subject to a periodical variation, in cighteen years seven months, owing to the nutation of the earth's axis.
Article VII gives the discovery and theory of the satellites of the planets Saturn, Jupiter, and Herschel. The satellites have been useful in calculating the masses of their respective primary planets. Those of Jupiter, in particular, have also been of eminent service, in determining the progressive motion of light, and its velocity, as well as the longitude of places on the earth. It is not to be wondered at, therefore, that the most eminent astronomers have assiduously applied themselves to accurate ob. servations on these secondary planets, and to a construction of tables for computing their motions and places; as Galileo, Reineri, Marius, Peiresc, Hodierna, Borelii, Cassini, Bremer, Maraldi, Bradley, Wargentin, Bailly, Euler, Lagrange, Laplace, Delambre, Herschel.
Article VIII discusses the subject of comets. These have been chiefly interesting, since it was announced by Newton, that comets are a kind of bodies which revolve about the sun like planets, but in very long or eccentric orbits. In consequence, Dr. Halley first predicted the time of the return of a comet, which it accordingly fulfilled in the year 1759; and this is the only comet which has yet answered to prediction. The ·labours of the chief astronomers who have written on the subject of comets are here minutely described.
In the ninth article, the history treats of those comets which can so nearly approach the earth, as to produce danger of its destruction. This article seems to have been written, in consequence of the great alarm excited at Paris, in the year 1773, by a paper on the same subject then written by Lalande; when it was found necessary, in order to calm the public mind, for the lieutenant of police to require from that astronomer a memcir, to certify the public that there was no danger to be apprehended from any such kind of accident. The memoir, when written, was referred to M. Montucla, the official censor of books for that period, who gave his certificate to this effect: • I have read, by order of the chancellor, a manuscript, entitled, “Reflexions on such Comets as can approach the Earth,” and I have found nothing in it that can authorise the imaginary terrors concerning such approximation; while, on the contrary, the pamphlet secms rather calculated to appease them, by showing that such a dreadful accident, though a thing possible, is of that order of possibilities to which no reasonable being pays any attention, on account of the extremely small degree of chance of its taking place, according to the laws of probability. • 'The tenth article treats of the libration of the moon, of our poles of rotation, and of the singular circumstance by which she revolves once round her own axis. in exactly the same time as she fulfils her period round the earth.
The two remaining articles of this book are on the flux and reflux of the sea ; in which, after slightly noticing the vague * ideas of former philoscphers upon this subject, of the ancients, of Descartes, Galileo, Baliani, Wallis, &c., M. Montucla advances to an explanation of the true cause, or that which consists in the mutual attraction between the earth and the two lu· minaries—the sun and moon. He then adds, that, on the same principles, more particular and ample explanations have been given by D. Bernouilli, Maclaurin, Euler, d'Alembert, and Laplace.
The History now enters on the seventh book of this part of the work, which contains, in so many articles, the history of - astronomical tables, of ephemerides, of calendars, of instru. ments, of observatories, and of judicial astrology. The first and second articles are simply catalogues of astronomical tables · and of ephemerides, with sometimes a line or two of remarks upon them, of little consequence.
The third article treats of the Gregorian calendar. The first reformation of the Julian calendar, by pope Gregory XIII, and its adoption by the catholics, was treated of in the first volume of this History. In the present article, therefore, the historian only adverts to the time and manner of adopting the correction and change, by the protestant states of Europe, with the contests that occurred on the occasion. The new calendar was first admitted into Germany, Switzerland, Denmark, and Sweden, in 1700; but into England, not till the year 1752; since which time, a difference of 11 days continued between the old and new style, till the year 1800; and, since then, another intercalary day being at that period omitted, the difference between the two styles has been twelve days, and will so continue for several centuries to come.
The Russians, who are of the Greek church, still adhere to the old style; and it is remarkable that the present History does not in any way advert to this circumstance.
Article IV treats on the new French calendar, though in a very obscure and imperfect manner. It seems to have been the joint work of Lalande and d’Eglantine, and to have been regulated by the following positions: viz. 1. that the æra should commence from the foundation of the republic, or the 22d of September, 1792: 2. that each year should begin at midnight with the day of the true autumnal equinox for the observatory of Paris: 3. that the year should consist of 12 months of 30 • days each, with five supplementary days at the termination of common years, and six of the bissextile.
The fifth article enumerates the various astronomical instruments that have been principally employed; such as gnomons, sectors, mural arcs, whole circles, theodolites, transit and parallactic instruments, equatorials, &c., with the names of the principal makers; among whom, of course, the English are the most conspicuous.
Observatories naturally follow from this subject; and they are noticed in the next article. Some account has been already given, indeed, in the second volume, of the two grand establishments of this kind in France and in England. But the eighteenth century has been very fruitful upon this point, as appears by an enumeration here made of the observatories constructed in all the countries of Europe, as well as in some parts of Asia and America.
In the seventh or last article of this book, a short account is introduced of the vain and pretended science of astrology. False and frivolous as this art may be, the author holds himself excusable for giving it a place, from the benefit it has produced to the true science of astronomy, and indeed to almost all the other mathematical sciences. It is no less certain than extraordinary, that the astronomy of all countries and all ages, from the Chaldeans and Egyptians down to the eighteenth century, was either judicial astrology, or a science cultivated for its benefit; and to excel, even in this kind of astronomy, it is manifest that the preparatory branches of arithmetic, geometry, and other sciences, must have been studied. We find, in this article, curious notices concerning many remarkable characters who have figured in this line. Happily the knowledge and progress of true science, particularly of the true system of the earth's motions, about its axis and round the sun, have freed the astronomers of Europe (though not all the common people) from the false allurements of astrology. Yet the Turks and the eastern nations generally still hold the fallacious notions of
antiquity; and even think, whatever we may pretend, that we no less cultivate astronomy for the sake of the occult science of astrology, than we ransack the ruins in their countries in search of lost or hidden treasures. .
The next or eighth book contains the history of the progress of navigation in the eighteenth century, with regard both to the construction and management of ships. The first article treats of their construction, which attained its greatest improvement in the course of the century adverted to. A particular account is subjoined of the treatises that have been written on this branch of naval architecture, with the institutions, or acts of governments, or other plans for its improvement, projected in different countries : whence it appears that the French have paid incomparably more attention to it than the English; and that, while it has been with the former a great national object, with the latter it has been left to chance, or to the care of indi. viduals : that the only society or association of this kind in England, which was formed in 1791, and wore the most promising aspect, has been suffered to languish and expire, through the total want of public encouragement. Hence it is not to be wondered at, that, in the article of ship-building and theoretic navigation, the French have far surpassed us.
The second article here treats of the arched or curved shape of the hull: article III, of the oars ; article IV, of the sails and inasts, with the action of the wind upon them : article V, of the management or working of ships. In this part, much ingenious mechanism is necessarily considered; such as the resistance of the hull in passing through the water; the action of the wind against the sails, with the manner of placing or setting these, to have the best effect; also, the position and manage ment of the helm ;--subjects which have exercised the ingenuity and learning of some of the ablest mathematicians, as Ranau, Huygens, Bernouilli, Euler, Bouguer, Clairaut, Juair, &c.
Article VI treats on the helm, which, it is well known, is a very important particular, as well for velocity of sailing, as for directing and varying the ship’s motions, especially in time of action. The longer the ship, the more difficult in turning by the helm; and it is here conceived to be owing to the greater length of the French ships than of the English, that the latter, by manoeuvring better, have the advantage in sea engagements.
The seventh is a very important article, and relates to the rea sistance of fluid to solid bodies, when moving through them. A neat abridged account is here given of the chief experiments and theories that have been made and laid down for obtaining the resistance of fluids to bodies of different shapes and in different circumstances, and for applying them to the improvement of naval architecture. These experiments and theories are chiefly those of the philosophers on the continent, such as Bouguer, Juan, Borda, Bossut, Dumaitz, Thevenard, Romme, who have all of them laboured and written very usefully on this important and public concern. The historian might, however, have availed himself of the labours of some philosophers in our own island, particularly those of the late society for the improvement of naval architecture, who made some of the best experiments, and drew from them the most useful practical rules. The chief results noted by the historian are, that the vessel sails better, or faster, when built pretty large or thick at the prow, and drawn out long and narrow behind; also, that, with the same breadth, the prow may be considerably varied in dimensions, without altering the velocity or rate of sailing; and that it is better to make experiments on the vessels themselves, than on small models.
In the eighth article is introduced a very sensible paper on the irregular motions of a ship, called rolling and pitching, the former being the motion from side to side, and the other from end to end; in which are contained the results that have been given, in consequence of government encouragement, or directions of several learned men and mechanists, to obtain for ships under sail the greatest degree of stability.
The ninth article creats on the best distribution of the loading of a vessel, the various positions of which, it is well known, make a very great difference in a vessel's sailing. The Academy of Sciences have published many pieces on this subject, received in consequence of rewards held out to learned men for that purpose, as Euler, Bossut, Groignard, Gauthier.
The tenth article is wholly employed on an account of that excellent book, the Examen Maritimo of don Jorge Juan, formed on a happy conjunction of theory and practice.
Article XI is devoted to the writings relating to the gauging or measuring of vessels, in order to ascertain their tonnage; the twelfth to the cordage; and the thirteenth, and last, to the modes of procuring sweet and wholesome water from the salt water of the sea, by means of distillation.
To the above succeeds the ninth or last book of the history of navigation, which contains the progress of the art of pilotage, or that which respects the course and situation of a ship. Of this part, the first article treats on the mariner's compass. From the history of this instrument, it appears that it was used in Europe in the twelfth or thirteenth century." An account is given of the changes of the magnetic direction, as to declination and inclination, as also of the construction of artificial magnets; in the course of which, Lalande, with his accustomed partiality, ascribes the chief merit to his compatriots, slighting and disparaging the ingenious labours of Knight, Mitchel, Canton, &c. °